1. Field of the Invention
This invention relates to a lithium ion secondary battery.
2. Description of the Related Art
As electronic devices have been reduced in their size and become portable, a nonaqueous electrolysis secondary battery, particularly a lithium ion secondary battery has been frequently used as a driving source for a portable electronic device such as a portable phone and a mobile terminal.
Furthermore, rapid increase of automobiles has caused serious global problems such as air pollution, global environmental pollution including discharged carbon dioxide and energy depletion. An electric car has become promising for improving these problems.
A battery for an electric car must meet requirements for battery properties such as an output density and a long life as well as an energy density.
There is substantial interest in a lithium ion secondary battery because of its improved output properties and its light weight.
Common characteristics in lithium ion secondary batteries will be described below.
Advantages
1. a higher energy density;
2. a higher output density;
3. improved high-temperature performance (efficient discharge/charge because of a reduced capacity loss or heat generation even at an elevated temperature);
4. reduced self-discharge (reduced capacity loss during storage);
5. absence of memory effect (no problems in repeated partial discharge).
Disadvantages
1. weakness to overcharge/overdischarge (a protection network is necessary);
2. an electrolytic solution comprising an organic solvent as a main component (a protection system is needed for ensuring safety during an abnormal state).
These are common characteristics in lithium ion secondary batteries.
A cathode (positive electrode) in a lithium ion secondary battery comprises a cathode activator such as LiCoO2, LiMn2O4 and LiNiO2 which can occlude and release lithium.
Among these, lithium manganese spinel (LiMn2O4) having a spinel structure is suitable to a battery for driving an automobile requiring a large capacity because it shows higher stability in a charged state than other lithium-containing complex oxide cathodes.
It is known that it has a smaller capacity in a 4 V range than other lithium-containing complex oxide cathodes and that its battery capacity is considerably reduced in the course of repeated charge/discharge. Thus, it is essential to improve the charge/discharge cycle properties.
An anode (negative electrode) comprises an anode activator such as carbon materials, lithium complex oxides, metals which can form an alloy with lithium, lithium metal or their mixtures, which can occlude or release lithium.
Among these, crystalline carbon materials such as coke and graphite and amorphous carbons such as non-graphitizable carbon (hard carbon) are used as an anode material because of absence of short-circuit with a cathode due to dendritic growth of electrodeposited lithium or lithium loss from an anode.
A highly crystalline graphite anode shows a higher discharge voltage and improved flatness in a voltage.
However, a current capacity per a unit weight of carbon (mAh/g) is 372 mAh/g is believed to be a theoretical upper limit because in a charged stage, lithium ions enter between graphite crystal layers of anode up to a ratio of one lithium atom to six carbon atoms.
On the other hand, non-graphitizable carbon (hard carbon) anode shows lower flatness in a voltage and continuous and gradual reduction of a voltage in the course of discharge. However, a crystallite orientation is random and a charge mechanism is different from that in a graphite anode so that a current capacity per a unit weight is higher than that in a graphite anode, and thus, an anode exhibiting more than 400 mAh/g has been developed.
Operation of an electronic device requires supply of a certain level of voltage, and a graphite anode showing a voltage with good flatness is preferable. In an automobile, a graphite anode showing a higher discharge voltage and higher flatness thereof is also suitable.
On the other hand, for a hybrid electric car in which an intermediate state of charge (SOC: State of Charge) is consistently maintained and frequently repeats a discharge/charge cycle, a non-graphitizable carbon anode showing gradual reduction in a voltage is rather suitable and advantageous because of easier charge control. Furthermore, a hybrid car requires that a regenerative charge during breaking is efficiently received by a small battery. Since a non-graphitizable carbon anode shows gradual reduction in a voltage depending on a discharge quantity, a battery using the anode can receive a large regenerative current.
A hybrid car using electricity and gasoline is not a ZEV (zero exhaust vehicle) like an electric car, but is a low-pollution car discharging a substantially less amount of harmful matters such as CO2 and NO2 than an existing car and showing a higher fuel efficiency. It has been, therefore, intensely developed and become considerably popular.
In terms of obtaining a large current capacity using a non-graphitizable carbon anode, Japanese Laid-open Patent Publication No. 1996-69819 has proposed an anode material in which a non-graphitizable carbon is coated with a graphitizable carbon or an anode material in which a graphitizable carbon is coated with a non-graphitizable carbon. However, since the anode material is a simple complex of a non-graphitizable and graphitizable carbon materials, it cannot fully solve the problem of volume variation in a graphitable carbon material associated with occlusion and release of lithium. Furthermore, a volume variation rate associated with occlusion and release of lithium is different between graphitizable carbon and non-graphitizable carbon. Therefore, as occlusion and release of lithium is repeated, detachment may occur in an interface between the graphitizable and the non-graphitizable carbons. In addition, the anode material requires heating at a high temperature of 2800° C., leading to a significantly higher production cost.
For solving the problem, Japanese Laid-open Patent Publication No. 2000-200603 has disclosed a carbon material consisting of three phases having different lattice spacings such that one particle can occlude and release ions between carbon-material layers.
In Japanese Laid-open Patent Publication No. 2000-200603, lattice spacings in a 002 plane are, for example, 0.3354 nm or more and less than 0.3375 nm in the first phase 11, 0.3375 nm or more and less than 0.3425 nm in the second phase 12, and 0.3425 nm or more in the third phase 13.
By the way, a lattice spacing in a 002 plane for graphite is 0.3354 nm, and the first phase 11 has a crystal structure similar to that of graphite.
Thus, the anode material has the combined properties of the first, the second and the third phases 11, 12, 13. Specifically, the anode material can show an improved density, a larger release capacity per a unit volume and an improved charge/discharge efficiency by the first phase 11. Furthermore, the anode material can provide a large initial occlusion capacity by the third phase 13.
Furthermore, the anode material has the first, the second and the third phases 11, 12, 13 in one particle. Significant volume variation in the first phase 11 during occlusion or release of various atoms or ions can be, therefore, absorbed by the second and the third phases 12, 13, resulting in improved cycle properties.
Table 1 shows the crystal conditions of the individual layers in the anode material. In the table, the comparative example has a low density of 1.46 g/cm3 and an average lattice spacing of 0.348 nm in a 002 plane and thus, is amorphous.
TABLE 1Lattice spacingin a 002 plane (nm)Crystallite size (nm)Density1st2nd3rd1st2nd3rd(g/cm3)phasephasephasephasephasephaseExample 11.670.3360.3420.344>100285Example 21.610.3370.3390.34386366Example 31.760.3360.3380.344>100567Comp.1.460.3482Example
In the publication, the anode material was used to prepare an anode, which was then used a secondary battery. The battery was prepared and an evaluation thereof was conducted as follows.
N-methyl-2-pyrrolidone as a solvent was added to a mixture of 90 wt parts of the anode material obtained and 10 wt parts of polyvinylidene fluoride as a binder to prepare an anode mixture paste, which was then evenly applied to a copper foil. After fully drying the anode mixture, it was peeled off from the copper foil and was then punched into a disk with a diameter of 15.5 mm to provide an anode.
After preparing the anode, it was used to prepare a coin form of secondary battery. In the process, there were used a cathode formed by punching a lithium metal plate with a thickness of 0.8 mm into a disk with a diameter of 15.5 mm; an electrolytic solution prepared by adding LiPF6 to a 1:1 (by volume) solvent mixture of ethylene carbonate and dimethyl carbonate in a rate of 1 mol/L; and a polypropylene microporous membrane as a separator.
The secondary battery thus prepared was charged/discharged to determine an occlusion capacity, a release capacity and a charge/discharge efficiency for the anode. In the process, charging was conducted by repeating a cycle that a constant current of 1 mA was applied for 1 hour and then current application was stopped for 2 hours until a battery voltage reached 4 mV. Discharging was conducted by repeating a cycle that a constant current of 1 mA was applied for 1 hour and then current application was stopped for 2 hours until a battery voltage reached 1.5 V. An occlusion capacity of the anode was calculated by dividing an electricity quantity during charging by the weight of the anode material contained in the anode. An release capacity of the anode was calculated by dividing an electricity quantity during discharging by the weight of the anode material contained in the anode. These were expressed in mAh/g. In addition, a charge/discharge efficiency was determined by multiplying a ratio of a release capacity/occlusion capacity by 100. The charge/discharge efficiency indicates how much lithium ions occluded between carbon material layers are efficiently used. These results are shown in Table 2.
TABLE 2Charge/OcclusionReleasedischargecapacitycapacityefficiency(mAh/cm3)(mAh/cm3)(%)Example 194779084Example 289270078Example 398681883Comp. Example64545370
As a casing of a secondary battery, a laminate packing film has been developed in place of a nickel-plated iron or aluminum canister.
In addition to a mobile device, weight reduction is inevitable in an automobile battery. Thus, the above film is believed to be the most suitable form of battery casing.
An aluminum laminate film for a battery basically has a three-layer structure consisting of a substrate 101/an aluminum foil 102/a sealant as shown in FIG. 1.
The substrate 101 is a film with a thickness of 10 to 25 μm of polyester (PET) or Nylon. The aluminum foil 102 is an aluminum film with a thickness of 20 to 40 μm. The sealant 103 is a film with a thickness of 30 to 70 μm made of polyethylene (PE), polypropylene (PP), modified polypropylene (PP) or an ionomer.
The aluminum laminate film for a battery is processed into a battery casing component by thermally sealing the sealant with a seal width of 2 to 5 mm at 160 to 180° C. for about 5 sec. as shown in FIG. 2.
The sealant is selected from those in which water permeation is smaller through a seal cross section from outside and which is less swollen with a carbonate solvent as an organic solvent for a electrolyte. Furthermore, an interface between the aluminum foil and the sealant layer may be protected such that the aluminum foil is not eroded by HF generated from LiPF6 as a lithium salt due to a trace amount of moisture present in the inside of the battery.
When using polyethylene (PE), polypropylene (PP) or modified polypropylene (PP) as a sealant, a water penetration is 300 ppm or less even under high-temperature and high-humidity conditions (60° C., 90% RH) for 100 days. Thus, the casing can be satisfactorily used as a battery packing component.
It has been believed that a secondary battery using a non-graphitizable carbon anode shows excellent temporal stability (storage property) of battery properties such as cycle properties and an electric capacity because a crystal is not expanded due to entering of lithium ions between graphite crystal layers during charging like a graphite anode.
It has been, however, found that storage properties may be deteriorated even in a secondary battery using a non-graphitizable carbon anode.
We have found that such deterioration in storage properties in a secondary battery using a non-graphitizable carbon anode is caused by a protrusion formed in a non-graphitizable carbon (amorphous carbon) anode.
We have also found that the protrusion may be grown to several microns, causing
1. microshort-circuit between the protrusion tip and a cathode; and
2. increase in a resistance (capacity reduction) associated with increase of a gap between a cathode and an anode due to protrusion forming.
The phenomenon is prominently observed when using a laminate film rather than a nickel-plated iron or aluminum canister as a casing.
In a metal canister casing, the phenomenon may be avoided because a pressure is applied.